Polypropylene is an inexpensive thermoplastic polymer employed in a wide variety of applications, the articles of which include, for example, films, fibers, such as spunbonded and meltblown fibers, fabrics, such as nonwoven fabrics, and molded articles. The selection of polypropylene for any one particular application depends, in part, on the properties of the polypropylene polymer candidate, the fabrication mode or manufacturing process and the final article and its intended uses. Examples of some of these properties include density, molecular weight, molecular weight distribution, melting temperature, and melt flow rate.
The final properties of polypropylene are generally dependent upon the polymerization conditions present during polymer formation. One such polymerization condition is the catalyst. In some instances, while the catalyst selection is an important component in the polymerization reaction, changing other polymerization condition variables in the presence of the same catalyst may produce polypropylenes having different final properties. For example, the addition of hydrogen to a metallocene catalyzed polymerization reaction may increase the catalyst activity. Catalyst activity may be measured by the increase or decrease in the amount of polymer produced during a measured time interval by a measured amount of catalyst. Generally, an increase in catalyst activity results in an increase in the amount of polymer produced by the catalyst over a measured time interval. Producing more polymer with the same catalyst or using less catalyst to produce the same amount of polymer may provide a commercial advantage.
However, there are many instances where the addition of hydrogen not only increases the amount of polymer produced but, in general, may also increases the melt flow rate (MFR) of the polymer. Many manufacturing processes have specific, in not strict, polymer melt flow rate parameters. Generally, high MFR (low molecular weight) polymers are not suitable in many applications. This is so because melting and handling the molten polymer are common steps for converting the polymer into a finished article. As such, a polymer having a high melt flow rate may, upon melting, become too fluid (or lack sufficient viscosity) to be processed into, for example, a foamed article, or an extruded fiber suitable for forming a nonwoven web or a thermoformed article. As such, increasing both polymer production and the polymer's melt flow rate may result in the production of greater quantities of polymer which are unsuitable for many manufacturing processes.
In other instances, it may be desirable that the finished polypropylene article possess a certain level of clarity. In some of these instances, clarity may be achieved by the addition of ethylene to the propylene polymerization process. The incorporation of ethylene into the polypropylene chain tends to break up or otherwise alter the polypropylene crystalline structure. The incorporation of ethylene in some instances may also reduce the molecular weight of the polymer and increase the melt flow rate. Again, because many manufacturing processes have strict melt flow rate parameters, increasing the melt flow rate of the polymer may not be desirable.
Additionally, certain catalysts, and particularly certain metallocene catalysts are suitable for producing polypropylene having a melt flow rate in a range of from greather than 19–2,000. Generally, a catalyst and particularly a metallocene catalyst, capable of producing polypropylene and particularly homopolypropylene having a melt flow rate greater than 19 may be referred to as a high melt flow rate polymer producing catalyst. Polypropylene polymers having a melt flow rate in this range may be useful in some applications, such as fiber spinning, melt blowing, injection molding and hot melt adhesives. If, however, polypropylene having a melt flow rate below this range is desirable, the polymer producer may, in some instances, be required to use a different catalyst. Switching catalyst may not only be expensive but time consuming as well.
Therefore, while it is desirable to improve polymer production and polymer properties, such as clarity, there exist a need to achieve these objectives while remaining compliant with processing melt flow rate parameters. Additionally, there exist a need to expand the melt flow rate capability of high melt flow rate polymer producing catalyst to avoid the associated cost and time inherent in switching catalysts.